In general, electrophotographic photoconductors contain an aluminum drum serving as a conductive support, and a photosensitive layer formed thereon. The photosensitive layer often contains a plurality of layers. In this case, for preventing a lower layer from dissolving in the formation of an upper layer, the lower layer is formed as a three-dimensionally crosslinked cured film in many cases. For example, penetration of a charge generation or transport material into an under layer causes leakage of charges, and thus, the under layer is formed as a film insoluble to a solvent; i.e., a three-dimensionally crosslinked thermosetting resin layer.
Also, conventional electrophotographic photoconductors are consumable supplies which must be replaced due to abrasion over time. In recent years, development has been made on electrophotographic photoconductors which have improved abrasion resistance leading to longer service lives, in attempts to decrease the frequency of replacement and increase the number of sheets printed using one electrophotographic photoconductor to the greatest extent possible. For example, in order to improve the photosensitive layer in mechanical durability, a densely, three-dimensionally crosslinked protective layer is formed on the surface of the photosensitive layer (see, for example, PTLs 1 and 2).
For increasing the mechanical strength or heat resistance of a charge transporting part in electrophotographic photoconductors, a charge transporting material and a binder resin are advantageously combined together as a single material.
Thus, there are proposed a charge transporting monomer formed by introducing a radical polymerizable group into a charge transporting structure and a polymer of the monomer, and an acrylic acid ester having a triphenylamine skeleton and a polymer thereof (see, for example, PTLs 3 and 4).
Also, some patent literatures propose that charge transporting monomers each having two or more radical polymerizable groups are applicable to electrophotographic photoconductors for forming a three-dimensionally crosslinked cured film, and also, propose a variety of such charge transporting monomers. In particular, there are disclosed a number of acrylic acid ester compounds exhibiting excellent crosslinking property, (see, for example, PTLs 5 and 6).
These charge transporting monomers can be used to form a crosslinked charge transporting material having high abrasion resistance. However, they require a polymerization initiator, or irradiation of UV rays or electron beams for crosslinking reaction. Thus, it is unavoidable that the obtained crosslinked charge transporting material is degraded in electrical characteristics.
Furthermore, once conventionally used cured films have a crosslinked structure, they lack thermoplasticity and dissolvability. As a result, these cured films cannot be recycled, and must be disposed of through combustion or landfilling.
Meanwhile, in consideration of resource saving, electrophotographic photoconductors have been increasingly recycled. Especially, attempts have been made to recycling of aluminum drums (i.e., conductive supports). In this case, the photosensitive layers are removed for recycling by generally washing them off with a solvent or peeling them off through swelling. However, the above-described three-dimensionally crosslinked cured, insoluble films on the photoconductors are not swellable or soluble with respect to a solvent, and are quite difficult to remove. Removal of the cured film with a knife or other tools tends to damage the aluminum drum, and even minor scratches prevent the aluminum drum from being recycled.
As described above, electrophotographic photoconductors containing a three-dimensionally crosslinked structure in a photosensitive layer have been increased in recent years, but become difficult to recycle. There is no electrophotographic photoconductor which has such a crosslinked structure and can be easily recycled.
In the meantime, Diels-Alder reaction may proceed in the absence of a catalyst, and is known to a reversible reaction involving a less amount of by-products. There are reports on the synthesis of resins through this reaction (see, for example, PTL 7 and NPL 1). Thus, if a charge transport layer formed through such Diels-Alder reaction can be used in electrophotographic photoconductors, the obtained electrophotographic photoconductors are expected to be excellent in abrasion resistance and charge transportability, and to be easily recycled with less environmental load.
However, such materials and electrophotographic photoconductors have not yet been known.